This invention relates to an x-ray mask for use in the manufacture of semiconductor devices by x-ray lithography. More particularly, it relates to an x-ray mask which employs a Ti-W alloy as an x-ray absorber pattern.
Since the advent of x-ray lithography, the material used for absorber patterns of x-ray masks has primarily been gold. Gold has a high absorption coefficient with respect to the soft x-rays (such as those corresponding to the PdL.alpha., MoL.alpha., and SiK.alpha. lines) which are used in x-ray lithography. Furthermore, as gold is soft compared to other metals, when a gold film is formed on a mask substrate, the stresses which are imparted to the substrate, which are the cause of distortions in the absorber pattern, are low.
However, gold is chemically stable, and while it has good resistance to chemicals, it is not possible to pattern a gold film using reactive ion etching, which is now much employed for the processing of semiconductor devices. Instead, it is necessary to pattern a gold film by ion sputter etching or by the lift-off method using electroplating. Ion sputter etching is a method in which gold is sputtered by an argon plasma and etching is performed. In this method, the sputtered gold is redeposited on the mask substrate which is being processed, and therefore an absorber pattern with vertical edges can not be obtained. On the other hand, the lift-off method using electroplating involves a step in which a resist pattern for lift-off is formed and a step in which electroplating is performed. It is therefore complicated, and as plating is a wet process, the resulting pattern has many defects.
In recent years, tungsten has come to be used as an alternative to gold as a material for x-ray absorber patterns. Tungsten has approximately the same x-ray absorption coefficient as gold, and it has the advantage that a pattern with vertical edges and a width on the order of 1 micron can be achieved by reactive ion etching with a conventional reactive gas such as CF.sub.4.
However, a tungsten absorber pattern has the drawback that it has poor adhesion to inorganic compounds such as SiN, SiO.sub.2, and BN which are commonly used as the substrates for x-ray masks, and the absorber pattern may peel off the mask substrate when the mask is washed or otherwise processed subsequent to the formation of the absorber pattern.